1. Field of the Invention
The invention relates to a light source device for a dielectric barrier discharge lamp. In particular, the invention relates to a light source device for a dielectric barrier discharge lamp which is used as an ultraviolet (UV) light source for a photochemical reaction, and in which light radiated from excimers formed by the dielectric barrier discharge is used.
2. Description of the Related Art
Conventionally, when controlling light from a light source device of a dielectric barrier discharge lamp, two processes are typically performed, either controlling the voltage applied to the lamp or controlling the frequency of the voltage applied to the lamp. In the process in which the voltage applied to the dielectric barrier discharge lamp is controlled, there is an upper limit with respect to the radiant efficiency of the UV radiation and a lower limit with respect to the uniformity of emission. For example, if a dielectric barrier discharge lamp in which a fluorescing material has been applied to the inside of the discharge vessel is used for purposes of image processing, the region between the these upper and lower limits is narrow. Therefore, adequate light control by this process is not possible.
In the process in which the frequency of the voltage applied to the lamp is controlled, light control is possible in a relatively wide range. There is a disadvantage, however, of reducing emission uniformity when the frequency is reduced and the amount of emission is decreased. In order to avoid this reduction in the uniformity of emission, the voltage applied to the lamp is controlled such that uniformity of emission is ensured in a state in which the emission amount is small. Conversely, when the frequency is raised and the amount of emission is increased there is a disadvantage of reducing the radiant efficiency of the UV radiation.
Japanese Patent Disclosure Document HEI 11-233071 describes a light source device for a dielectric barrier discharge lamp in which a supply means is provided, the supply means being divided into two parts and in which the frequency of the AC voltage and the DC source voltage are controlled. The object of this device is to change the voltage in the area in which reduction of the radiant efficiency of the excimer emission is allowable as a result of the increase of the voltage applied to the lamp. This is performed by uniform irradiation of a body to be irradiated with a large area by several dielectric barrier discharge lamps, with respect to voltage control. In addition, with respect to the frequency setting, the device carries out precision adjustment of the nonuniformity as a result of scattering of the radiant efficiency of the individual lamps by several lamps. In this device, the voltage applied to the lamp and the frequency are controlled without any connection to one another and independently of one another. Moreover, problems with respect to the uniformity of emission and the radiant efficiency in the individual dielectric barrier discharge lamps are neither recognized nor indicated.
The following is a description of problems associated with light control for a light source device of a dielectric barrier discharge lamp. Normally, in a dielectric barrier discharge lamp, an electrical charge forms an electrical field which is moved by a discharge in a discharge space and has been deposited on a dielectric. The effect is used by superposition of this electrical field on an electrical field which is formed by a voltage applied from outside the lamp, the outside voltage necessary for starting the discharge is reduced essentially by half.
In a case, however, in which the period after formation of a discharge and movement of the electrical charge until starting of the next discharge, the voltage applied from the outside to the dielectric barrier discharge lamp is changed, the electrical charge present on the dielectric is moved by electrical conduction of residual plasma in the discharge space after completion of the discharge and neutralized. This phenomenon is inevitable in the light source device of a dielectric barrier discharge lamp with a feed device by which an AC high voltage applied to the dielectric barrier discharge lamp is generated by a step-up transformer. The reason for this is that a step-up transformer cannot produce a strict DC voltage.
Normally, the voltage formed on the secondary side of a step-up transformer has a tendency to be continuously attenuated in the direction to zero voltage. Moreover, the voltage begins with an oscillation at the resonant frequency which is fixed by an inductance of the step-up transformer and the electrostatic capacitance of the dielectric barrier discharge lamp. The voltage is also changed in an oscillating manner by the voltage applied from the outside to the dielectric barrier discharge lamp as a result of the xe2x80x9cringingxe2x80x9d phenomenon, when the resonant frequency is higher than the control frequency.
Thus, in a case of reducing the emission amount by a reduction of the control frequency for light control, the time interval of a discharge compared to non-light control increases. Accordingly, the amount of electrical charge increases which is moved by electrical conduction of the residual plasmas of the discharge space after completion of the discharge which is present on the dielectric and is neutralized. The intensifying action of the electrical field which is formed by the electrical charge adhering to the dielectric, with respect to the electrical field which is formed by the voltage applied from outside the lamp, is changed. This means that the discharge intensity for non-light control and for light control changes even if the voltage amplitude of the voltage applied from outside the lamp for non-light control and light control does not change. This situation is described in FIGS. 10, 11(a) and 11(b).
FIG. 10 shows a schematic of one example of a light source device of a dielectric barrier discharge lamp. This device includes an invertor of a full bridge system. Reference number 1 labels a dielectric barrier discharge lamp to which a chopper voltage generated by switching devices Q91 through Q94 and a step-up transformer T91 is applied. In the switching devices Q91 through Q94, a voltage supplied by a power source US is subjected to gate control by gate voltages Vg1 and Vg2. Thus, a dielectric discharge is carried out.
FIGS. 11(A) and 11(B) each show the voltage waveform on the two ends of the dielectric barrier discharge lamp 1. FIG. 11(B) shows a case in which the control frequencies of the gate voltages Vg1 and Vg2 are made lower than those shown in FIG. 11(A). In this case, the time interval T1 in which the switching devices Q91 through Q94 are in the ON state does not change. With respect to the time interval in which all switching devices Q91 through Q94 are in the OFF state, as shown in FIG. 11(A), there is a short time interval T2a which changes in FIG. 11(B) into a long time interval T2b.
If the voltage of the power source US does not change, the voltage waveforms in the time interval T1 in FIGS. 11(A) and 11(B) have similar shapes. The amplitude Vp of a lamp voltage Ve shown in FIG. 11(A), therefore, has roughly the same value as that shown in FIG. 11(B). However, Vta shown in FIG. 11(A) and Vtb (Vta is larger than Vtb) shown in FIG. 11(B) label a voltage immediately prior to the lamp voltage Ve becoming negative by the switching devices Q92 and Q93 being turned on in the next half period. This is because in the interval in which all the switching devices Q91 through Q94 are in the OFF state, as a result of the LC resonant phenomenon, the lamp voltage Ve changes due to the electrostatic capacitance of the dielectric barrier discharge lamp 1 and to the inductance on the secondary side of the step-up transformer T91. Thus, in FIGS. 11(A) and 11(B), the interval T2b is larger than the interval T2a, and therefore, the amount of change of the lamp voltage Ve in FIG. 11(B) is greater than that in FIG. 11(A). This means that the electrical charge deposited on the dielectric in FIG. 11(B) according to the amount of voltage which is formed by (voltage Vtaxe2x88x92voltage Vtb) has been moved and neutralized more than that in FIG. 11(A). The strength of the discharge which is formed when the switching devices Q92 and Q93 are turned on is less in FIG. 11(B) than in FIG. 11(A).
In FIGS. 11(A) and 11(B), xe2x80x9cringingxe2x80x9d is formed in the interval T1 in which the switching devices Q91 through Q94 are in the ON state, as a result of the LC resonance by the electrostatic capacitance of the dielectric barrier discharge lamp 1 and due to the cross inductance of the primary winding and the secondary winding of the step-up transformer T91. Since the cross inductance is typically small, its resonant frequency becomes high. Therefore, in the case in which all the switching devices Q91 through Q94 are turned off in some phase of a state in which the amplitude of this resonance is high, and in which immediately the switching devices Q92 and Q93 are turned on, there are also cases in which, depending upon the phase, the discharge becomes stronger, the lower the frequency is made. This means that by changing the control frequency of the switching devices Q91 to Q94, the discharge intensity changes for non-light control and light control, even if the amplitude of the voltage applied from outside the lamp for non-light control and for light control does not change. When the discharge intensity decreases for non-light control, this change reduces the uniformity of emission and conversely, when the discharge intensity increases for light control, reduces the radiant efficiency. It can, therefore, be understood that in the case of a change of the control frequency for light control, it is necessary to change, and thus, adjust the amplitude of the voltage applied from outside the lamp in conjunction with the control frequency.
The present invention was devised to eliminate the above-described disadvantages with respect to light control in conventional dielectric barrier discharge lamps. An object of the invention is to devise a light source device of a dielectric barrier discharge lamp in which, for a large emission amount, light control is carried out by accomplishing an optimum state with respect to the uniformity of emission and the radiant efficiency of the UV radiation.
Another object of the invention is to devise a light source device of a dielectric barrier discharge lamp in which, for a small emission amount, the conventional disadvantage of a reduction in the uniformity of emission can be eliminated.
The above objects are achieved in accordance with a first embodiment of the invention by providing a light source device for a dielectric barrier discharge lamp which includes a dielectric barrier discharge lamp having a discharge space filled with a discharge gas which produces excimers by a dielectric barrier discharge and in which there is a dielectric between at least one of the two electrodes by which a discharge is to be induced in the discharge gas; a feed device for applying an essentially periodic AC high voltage to the electrodes of the dielectric barrier discharge lamp, the feed device having a setting mechanism which adjusts both the control frequency of the essentially periodic AC high voltage and the amplitude of the essentially periodic AC high voltage according to the set control frequency.
In a second embodiment of the invention, the feed device includes a power source, a voltage controller for controlling the feed voltage, an invertor which is triggered by the controlled voltage, an invertor switching device driver signal generating mechanism which produces signals for driving the switching devices for the inverter, a voltage control switching device driver signal generating mechanism which produces a signal for driving the switching device for voltage control, and a setting mechanism which adjusts both the control frequency of the essentially periodic AC high voltage and the amplitude of the essentially periodic AC high voltage according to the set control frequency. With respect to the invertor switching device driver signal generating mechanism, the setting mechanism outputs a set invertor control frequency signal. By the signals for driving the switching devices for the inverter, which outputs the invertor switching device driver signal generating mechanism, drives the invertor with the set control frequency. Moreover, with respect to the voltage control switching device driver signal generating mechanism, outputs a voltage setting signal. By the voltage control switching device driver signal, which the voltage control switching device driver signal generating mechanism outputs, adjusts the amplitude of the essentially periodic AC high voltage output by the voltage control mechanism in conjunction with the set control frequency.
In a third embodiment of the invention, the object is achieved similarly to that of the second embodiment, however, the setting mechanism includes a look-up table which outputs the correct data of a voltage setting signal, the data of the invertor control frequency setting signal being called addresses.
In a fourth embodiment of the invention, the object is achieved in providing a setting mechanism that, as the control frequency drops, adjusts the voltage amplitude such that voltage amplitude increases when the discharge intensity of the dielectric barrier discharge lamp decreases.